In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. Such a light-emitting element has a structure where a layer containing a material with a light-emitting property is interposed between a pair of electrodes. By application of a voltage between the pair of electrodes, light can be emitted from the material with a light-emitting property.
Characteristic features of the light-emitting element using electroluminescence can be found in its advantages in being manufactured to be thin and lightweight and having very fast response time. There are various possible applications of such a self-luminous light-emitting element. For example, the light-emitting element is preferably used for a flat panel display because it has advantages such as high visibility of pixels as compared to a liquid crystal display and ability to display bright image despite the absence of a backlight.
Furthermore, by applying the light-emitting element using electroluminescence, a planar light-emitting device with a large area can be easily formed. This is a feature which is difficult to obtain in point light sources typified by an incandescent lamp and an LED, or line light sources typified by a fluorescent bulb. In addition, such a light-emitting element has attracted attention as a preferable next-generation lighting device because it is estimated to have higher emission efficiency than filament bulbs or fluorescent bulbs.
The light-emitting elements using electroluminescence are roughly classified in accordance with whether they include an organic compound or an inorganic compound as a material with a light-emitting property. When the light-emitting element using electroluminescence uses an organic compound as a material with a light-emitting property, light is emitted in the following manner. First, a voltage is applied to a pair of electrodes between which a layer containing a material with a light-emitting property, in other words, an EL layer, is interposed, so that electrons and holes are transported from one electrode and the other electrode, respectively. The electrons and holes which are transported to the EL layer are recombined to form an excited state of the organic compound which emits light in relaxing to a ground state.
Such a light-emitting element which emits light by the transport of electrons and holes to the light-emitting layer is called a current excitation type light-emitting element. Note that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are many problems. Therefore, improvement in an element structure, development of a material, and the like have been carried out in order to solve the problems.
For example, in terms of improving emission efficiency, it is preferable that the light-emitting element realize a state where the numbers of electrons and holes which are transported to the light-emitting layer are balanced, i.e., a state where the balance of carriers is achieved, which achieves efficient recombination to result in increase in emission efficiency.
As an example of a known method for achieving a well balance of carriers, there is a method for providing a hole-transport layer between an anode and the light-emitting layer in the light-emitting element, and an electron-transport layer between the light-emitting layer and a cathode in the light-emitting element. Another method for achieving a well-controlled balance of carriers transported to the light-emitting layer includes the adjustment of the carrier-transporting abilities and thicknesses of the hole-transport layer and the electron-transport layer.
In Non-Patent Document 1, a method for preventing holes from leaking from a light-emitting layer to a cathode side with the use of a hole-blocking layer provided between the light-emitting layer and the cathode is disclosed. The holes are confined in the light-emitting layer, whereby the recombination of the electrons and the holes in the light-emitting layer is facilitated. As a result, the emission efficiency of a phosphorescent material is successfully increased.